ICs are widely used today in many different applications such as computers, cellular telephones and consumer electronics. ICs are typically fabricated from disc-shaped wafers of silicon on which large numbers of ICs are formed. One common method of IC fabrication is to deposit various layers of semiconductor material on the face of a wafer to form the ICs. The wafers are then cut so that the individual ICs can be separated and used for different applications.
"Dummy" or "control monitor" wafers are used to check the reliability of IC fabrication equipment by testing the quality of the ICs fabricated on the wafers by the equipment. For example, dummy wafers are used to test new IC fabrication equipment prior to its use in the actual production of ICs. A dummy wafer is cycled through the equipment and the ICs formed on the dummy wafer are then examined to determine if they meet certain specified criteria indicating that the fabrication process was properly performed. Only then can the equipment be used for the actual production of ICs. Thereafter, the dummy wafers are typically discarded.
Once fabrication equipment is in actual use producing ICs, it must be periodically inspected by examining the fabricated ICs to ensure that it is functioning properly. Such quality assurance testing is typically performed on a daily basis, sometimes as often as every working shift. In addition, such testing is also performed during regularly scheduled maintenance of the fabrication equipment. During such testing, silicon wafers are used as process monitors in which a trial process, such as film deposition, is performed on the wafer by the fabrication equipment. Prescribed measurements are then performed on the wafer to determine whether the fabricated ICs meet specified tolerances. Once these tests are completed the wafer is typically discarded.
Quality assurance testing of the type described requires the use of large numbers of silicon wafers. In fact, properly testing a single piece of fabrication equipment typically requires that thousands of control monitor wafers be used. This increases the total cost of IC fabrication because the wafers are relatively expensive and typically have no other use in the fabrication process. In order to defray some of the cost of these discarded wafers, they are sometimes recycled. Conventional recycling of wafers typically involves subjecting the wafer to several chemical cleaning steps to remove the deposited layers of semiconductor material. The chemical cleaning is then followed by a final chemical/mechanical polish. Once the recycling is complete, the wafers can be reused as dummy or control monitor wafers for non-critical operations, but they are not usually suitable for actual IC production.
Conventional recycling of wafers has a number of drawbacks. The chemical cleaning steps are relatively slow, thus adding to the overall cost of recycling. Also, the chemicals are expensive and constitute a significant portion of the recycling cost. In addition, the use of these chemicals creates potentially hazardous environmental handling and disposal issues that further increase the cost of recycling.
It is therefore an object of the invention to provide a method for reducing the overall cost of IC manufacturing. It is another object of the invention to provide a way to reduce the overall cost of recycling dummy and control monitor wafers used in the quality assurance testing of IC fabrication equipment. Another object of the invention is to provide a method for reducing the time it takes to recycle these wafers. It is a further object of the invention to provide a method for recycling such wafers that eliminates the use of potentially hazardous chemicals in the cleaning process.